1. Field of the Invention
This invention relates generally to an electrophotographic printing system, and more specifically, the present invention is directed to an improved image registration system.
2. Description of the Prior Art
The basic process of monocolor electrophotographic printing (e.g. black image placed on a white background) comprises exposing a charged photoconductive member. The irradiated areas of the photoconductive surface are discharged to record thereon an electrostatic latent image corresponding to the original document. A development system, thereupon, moves a developer mix of carrier granules and toner particles into contact with the photoconductive surface. The toner particles are attracted electrostatically from the carrier granules to the latent image forming a toner powder image thereon. Thereafter, the toner powder image is transferred to a sheet of support material. Following the toner image transfer to the sheet of support material, the support material sheet advances to a fuser which permanently affixes the toner powder image thereto.
Essentially, multicolor electrophotographic copying and printing (e.g. several colors placed on a white background) repeats the process of monocolor printing by repeating a plurality of cycles, each cycle being for a different color. Development stations for each of the different colors apply a specific color toner complimentary in color to the color of a filter utilized to produce the irradiated areas of the photoconductive member. The different color toners are generally, cyan, magenta, and yellow (and sometimes black if a true black is desired), which in one combination or another can be used to generate the full spectrum of visible colors.
Through the application of the different colored toners at the respective stations, a plurality of color toner powder images are formed for transfer directly to a sheet of support material or to an intermediate belt for subsequent transfer to a sheet of support material. In either case the images are transferred in superimposed registration with one another. After a plurality of different color toner powder images have been transferred to the sheet of support material in superimposed registration with one another, the multicolor toner powder image is permanently affixed thereto.
In recent years, there have been demands for providing high quality images. In order to achieve a full color reproduction using electrophotographic color printing, it is critical that the toner powder images be superimposed upon each other on the copy in near perfect registry. For example, image registration of at least 0.005 inches is required to minimize blur and color hue shifts.
In view of the importance of color reproduction capabilities and the emphasis on overall quality there is a need for an improved image registration system.
The following disclosures may be relevant to various aspects of the present invention:
______________________________________ US-A-4,188,110 Patentee: Stange Issued: February 12, 1980 US-A-4,401,024 Patentee: Frentress Issued: August 30, 1983 US-A-4,804,979 Patentee: Kamas et al. Issued: February 14, 1989 US-A-4,847,660 Patentee: Wheatley, Jr. et al. Issued: July 11, 1989 US-A-4,903,067 Patentee: Murayama et al. Issued: February 20, 1990 US-A-4,916,547 Patentee: Katsumata et al. Issued: April 10, 1990 US-A-4,935,788 Patentee: Fantuzzo Issued: June 19, 1990 US-A-4,963,899 Patentee: Resch, III Issued: October 16, 1990 US-A-4,965,597 Patentee: Ohigashi et al. Issued: October 23, 1990 US-A-5,016,062 Patentee: Alan E. Rapkin Issued: May 14, 1991 ______________________________________
The relevant portions of the foregoing disclosures may be briefly summarized as follows:
U.S. Pat. No. 4,963,899 discloses a method and apparatus for image frame registration in which registration indicia for registering an image frame are written on a photosensitive member in an interframe or frame margin area. A sensor array provides in-track and cross-track signal information to a control unit for synchronizing the electrostatic process of the registered image frames.
U.S. Pat. No. 4,916,547 discloses a color image forming apparatus which produces a single composite color image on a paper. The paper is transported by a belt and the composite color image is formed by transferring image components of different colors to the paper in register with each other. The apparatus reduces positional deviation of a plurality of image components of different colors by sensing signals on a surface of the transfer belt outside a paper region. The sensor senses arriving pattern images and corrects for unaligned images by calculating a deviation amount and adjusting a timing signal accordingly.
U.S. Pat. No. 4,847,660 discloses a method and apparatus for registration control in an electrophotographic printing machine. A plurality of separate image creation and developmental control signals are regulated by a timing clock which senses reference pulses generated upon arrival of a transfer belt at each reference station. The image creation can be temporally synchronized with the physical rotation of the belt assuring proper image registration.
U.S. Pat. No. 4,804,979 discloses a single pass color printer/plotter having four separate microprocessor-based print stations, each for printing a different color image for superimposition with one another to form a full color image. The printer includes a registration system where each print station monitors registration marks to correct for media variations. Each print station includes optical sensors that monitor the marks printed on the media edge to synchronize the printing and align the images properly.
U.S. Pat. No. 4,401,024 discloses a method and apparatus for establishing and maintaining registration control in a printing system which prints an image on a moving substrate. Registration marks are printed and sensed along the length of the substrate. These marks are used to correct for a misalignment that can occur during the transfer to subsequent print stations, thus, creating a clearer image.
U.S. Pat. No. 4,965,597 discloses a color image recording apparatus which superimposes a plurality of different color images on one another to form a composite image. Registration marks are formed on a recording medium and are sensed at each station to assure a clear and accurate superimposed image. A sensor senses one or both edges of a recording medium to note image deviations caused by transport to enable compensation thereof.
U.S. Pat. No. 4,903,067 discloses a multi-image forming apparatus in which image registration marks are formed to detect the position of different color images. The registration marks are formed on a transfer belt at regular intervals and separate from the different color images. CCD detectors sense the marks and accurately correct for any deviations that may occur along the transfer path.
U.S. Pat. No. 5,016,062 discloses an apparatus for forming multicolor toner images. A plurality of imaging members develop different color toner images. A transferring means transfers the different colored images in registration to either an endless web or receiving sheet carried by the web. The web contains perforations along an edge and a means is provided to maintain sprocket teeth within the perforations to maintain accurate registration.
U.S. Pat. No. 4,188,110 discloses a high speed color reproduction machine comprised of four separate xerographic processing units. A precise dimensional relationship, between the photoreceptor length and the spacing for each processing unit, assures registration of produced color images with one another.
U.S. Pat. No. 4,935,788 discloses a multicolor printing system in which a plurality of different color developed images are transferred to a conveying member in superimposed registration to form a multicolor image.